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Friday 9 October

Optics

Chiroptical Nonlinearity Found in Covalent Organic Frameworks

Yile Zhang, Lei Wei, Sixue Tang et al. (+4 more)

Chiroptical nonlinearity is vital for modern optics and photonics. However, it remains a central challenge to obtain a material simultaneously with large second-order nonlinearity and strong nonlinear optical chirality. Here we uncover the strong chiroptical nonlinearity in COF-330-a, owing to the formation of unique chiral, polar structure with large p-electron conjugation via structural transformation. A large effective second-order nonlinear susceptibility of ~ 101 pm/V and a strong nonlinear chiroptical response (SHG-CD: -1.60 to +1.66) have been demonstrated in individual COF-330-a microcrystals, transcending the performance trade-off limitation in conventional chiroptical materials. These findings open a new avenue for designing nonlinear chiroptical materials which harness the duality of polarity and chirality and chemical versatility of COFs.
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All authors Yile Zhang, Lei Wei, Sixue Tang, Yuhang Li, Yanhang Ma, Yue-Biao Zhang, Xiaoqin Shen

Optics · Cross-listed from quant-ph

Judd-Ofelt analysis and coherent spectroscopy of europium-based molecular complexes for quantum information applications

Nicholas Lester Jobbitt, Sascha Weis, Timo Schweizer et al. (+9 more)

Rare-earth-ion based molecular crystals are emerging as a promising platform for quantum information applications, combining long optical coherence times with the chemical tunability of coordination chemistry. Here we present a comprehensive optical characterisation of twelve Eu$^{3+}$-based molecular complexes, with a focus on the coherent $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ transition at $\sim$580 nm. Judd-Ofelt analyses are performed across all complexes, providing insights into ligand-field environments, local point-group symmetries, and the resulting branching ratios of the coherent transition. We find that lower point-group symmetry (C$_1$, C$_2$) yields higher branching ratios of the $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ transition, exceeding 1% in several complexes. Optical inhomogeneous linewidths measured at 4.2 K reveal narrow linewidths for some complexes of below ten GHz, indicative of a highly ordered crystalline environment. Optical coherence measurements of one of the complexes, 5-Eu, yields an optical coherence time of $T_{\text{2}}^{\text{opt}} = 2.3~\mu\mathrm{s}$ at 4.2 K, comparable to the best-performing molecular systems reported to date. Systematic characterisation and quantitative comparison will be key to find structure-property relations guiding rational design of optimised complexes for quantum information applications.
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All authors Nicholas Lester Jobbitt, Sascha Weis, Timo Schweizer, Vishnu Unni Chorakkunnath, Evgenij Vasilenko, Tobias Haas, Barbora Brachnakova, Senthil Kumar Kuppusamy, Hartmut Schubert, Mario Ruben, Michael Seitz, David Hunger

Optics

Aubry-André transition in photonic time quasicrystals

Shuhang Chen and Qi Zhu

In one dimension, Anderson localization by a disordered potential sets in at any strength, whereas localization by a quasiperiodic potential in the Aubry-André model occurs through a transition at a critical strength. Disordered photonic time crystals exhibit a temporal counterpart of this Anderson localization, in which light, instead of decaying in space, stops propagating and grows in time. Whether a counterpart of the Aubry-André transition exists in photonic time quasicrystals remains an open question. Here we show that it does, and that the transition occurs at mobility edges in momentum rather than in energy. These edges arise even under impulsive modulation, where the field obeys the self-dual almost-Mathieu equation exactly, and for general analytic modulations Avila's global theory reveals the condition under which they form. The Aubry-André transition thus extends from spatial lattices to the time domain, where it sorts light by wave number into components that propagate and components that stop and grow.
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Optics

UV Fluorescence Sensing of Medical-Chemical-and- Biological Analytes by an AlGaN/Si-on-Sapphire Optoelectronic Chip

Richard Soref

This paper proposes and analyzes a compact fluorescence-photometer lab-on-a-chip that is simpler than a fluorescence spectrometer, and that does not require spectrum reconstruction by a computer. An on-chip AlGaN VCSEL lasing at 280 nm, or at 310 nm, pumps an on-chip sample in the form of liquid or solid or tissue, thereby exciting signature fluorescence in the 300 to 540 nm wavelength range. This emission signal is detected and read out by an on-chip UV-enhanced Silicon photodiode. The 2 cm x 5 cm chip consists of C-plane (0001) Sapphire, and no lenses, prisms, or mirrors are utilized in the photometer. The chip is opto-electronic and features co-packaged electronics using tiny electronic chiplets that are bonded to the sapphire. These consist of an LD driver and transimpedance amplifier for the UV photonics, together with ASICs, ADC, microcontroller, micro-battery and RF-wireless or Ethernet chiplets. The mission of this self-contained cost-effective chip is sensing targeted medical-chemical-biological (MCB) agents in a local or network setting. Eight significant MCB applications are detailed here.
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Optics

Spectral-control-assisted Kerr-lens mode-locking in a Tm:Y2O3 ceramic laser

Zeyu Gong, Jing Wang, Panqiang Kang et al. (+5 more)

We report a soft-aperture Kerr-lens mode-locked Tm:Y2O3 ceramic laser stabilized by suppressing wavelength competition under high population-inversion conditions. The laser delivers an average output power of 0.5 W with a pulse duration of 112 fs at a repetition rate of 90 MHz and a central wavelength of 2.08 um. Further optimization for shorter pulses yields a pulse duration of 107 fs, through with a reduced output power of 0.22 W. These results demonstrate that spectral-control is an effective approach for achieving stable mode-locking in such laser with strong spectral competition.
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All authors Zeyu Gong, Jing Wang, Panqiang Kang, Weichao Yao, Yujie Peng, Jun Wang, Dingyuan Tang, Yuxin Leng

Optics

Localization Landscapes based on the Green's Matrix of Non-Hermitian Optical Scattering Systems

Luca Dal Negro and Hari Sundar

The Filoche–Mayboroda (FM) localization landscape predicts where the spatially localized modes of a self-adjoint elliptic operator reside without diagonalizing it, but the construction fails for the complex, non-normal Green's matrix G of an open scattering array of electric dipoles. We propose three real-valued, positive landscapes built from the Feinberg–Zee hermitization of G: a regularised Gramian inverse (Gram), the diagonal Schur block that carries the Metz–Neri–Rogers spectral density (SY), and the full hermitized inverse contracted against a polarization-isotropic source pair (Herm). The three form a progression from spectrum-blind to fully hermitisation-faithful, agree in the limit $z\to 0$, and diverge where it matters: at nonzero spectral shift SY resolves modes that Gram collapses, and Herm resolves modes that SY collapses. We demonstrate the progression on random and deterministic aperiodic scattering arrays with a wide range of structural correlations, extend to the vectorial case of transverse electric polarization (TE), and provide rigorous justifications along with mathematical proofs and a complete numerical recipe in the appendices and in the supplementary material.
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Optics

Bottom-Up Surface-Enhanced Raman Scattering for the Detection of Micro-Objects and Globular Analytes

Andrey K. Sarychev, Milena S. Shestopalova, Denis S. Korzhov et al. (+7 more)

Surface-enhanced Raman scattering (SERS) is typically achieved by depositing the analyte on a metal or semiconductor substrate. We propose a bottom-up approach, enabling SERS by depositing a metal on the surface of the object being studied. A Mie-theory analysis of electric-field enhancement in dielectric core-shell particles is presented. A correlative approach combining SERS, atomic force microscopy, and scanning electron microscopy is employed to model the conditions required for sensitive SERS detection and spectral analysis of polystyrene microspheres (PMSs) as models of globular and vesicle-like objects. Based on these findings, we propose a SERS-active system that provides strong signal enhancement (up to four orders of magnitude for a single globular object) while maintaining spectral fidelity, where PMSs act both as model analytes and as integral components of a metal-dielectric resonator.
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All authors Andrey K. Sarychev, Milena S. Shestopalova, Denis S. Korzhov, Andrey V. Ivanov, Igor V. Bykov, Konstantin N. Afanasev, Nikita N. Osipov, Dmitry V. Basmanov, Vladimir A. Oleinikov, Konstantin E. Mochalov

Optics · Cross-listed from nlin.PS

Solitons in the nonlinear Schrödinger equation with the pseudo-Raman term, anti-diffusion gain and nonlinear loss

Evgeny M. Gromov and Boris A. Malomed

We introduce a nonconservative nonlinear Schrödinger equation (NLSE) which includes additional nonlinear terms accounting for the pseudo-stimulated-Raman-scattering (pseudo-SRS) effect and cubic loss. An essentially novel feature of the model is the linear anti-diffusion gain, which leads to the obvious blow-up in the infinite or spatially periodic system. We consider the absorptive boundary conditions, which suppress the blow-up driven by effective convective instability in the setup. The NSLE model is derived, in the extended adiabatic approximation, from the underlying Zakharov system, which includes the viscosity term acting on its low-frequency component and the anti-diffusion gain in the high-frequency (HF) one. The latter term appears if the expansion, with respect to the deviation of the wavenumber, is performed not around the minimum-loss point (as in the usual situation), but around a maximum-loss point. Dynamics of moving solitons in the model is addressed by means of analytical and numerical methods. The sech approximation for the soliton predicts it as a stable fixed point (FP) of the system of balance equations for the HF norm and wave momentum, derived in the moving reference frame. The FP attraction basin is identified too. The predictions are corroborated by direct simulations of the full NSLE with the absorptive boundary conditions.
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Optics · Cross-listed from quant-ph

All-optical deconvolution via quantum cross-correlation spectroscopy with bright twin beams

Yujiro Eto

The resolution of spectroscopic measurements is fundamentally limited by the convolution between a system's intrinsic response and the instrument response function, creating an ill-posed inverse problem. While entangled photons can theoretically bypass these limits, low photon flux has hindered their experimental realization. Here, we propose quantum cross-correlation spectroscopy (QCCS), utilizing macroscopic twin beams (signal and idler beams) to physically circumvent this convolution limit in coherent anti-Stokes Raman scattering. Employing the signal beam as a broadband probe, QCCS achieves all-optical deconvolution through a correlation-enhanced sum-frequency generation channel between the convoluted anti-Stokes and idler beams. By subtracting the uncorrelated background using a 500-femtosecond temporal window, we successfully retrieve narrow Raman bands (8 cm^{-1} and 15 cm^{-1}) completely obscured by a 75 cm^{-1} probe bandwidth without numerical deconvolution or prior knowledge. Our results establish macroscopic quantum correlations as a robust resource for high-fidelity spectroscopy, paving the way for ultrashort-pulse-free operando studies under natural, incoherent excitation conditions.
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Optics

Matrix-free numerical continuation of the Ikeda map in passive Kerr resonators

Yuandi Xu, Pengxiang Wang, Jianxing Pan et al. (+2 more)

We introduce a matrix-free numerical continuation framework for the Ikeda map of coherently driven passive Kerr resonators. The framework traces solution branches and evaluates their linear stability without explicitly assembling the full Jacobian matrices, using a unified formulation for states that recur after one or multiple round trips. We resolve stable and unstable branches of modulation-instability patterns and temporal cavity solitons, and uncover period-doubled homogeneous states and associated multi-round-trip dynamics that are absent from the standard Lugiato-Lefever equation (LLE). For period-1 states, the Ikeda map and the LLE show close agreement in the high-finesse regime, while their branch-existence intervals increasingly deviate as the finesse decreases. These results demonstrate that direct continuation of the unaveraged round-trip map can capture nonlinear states and multi-round-trip dynamics beyond the mean-field description, providing a systematic framework for bifurcation and stability analysis of Kerr resonators without relying on the averaged LLE.
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All authors Yuandi Xu, Pengxiang Wang, Jianxing Pan, Haisu Zhang, Gang Xu

Optics

Silent breakdown of the perturbative polarization expansion in the terahertz regime

Yao Lu, Jiawei Zhang, Qiang Wu et al. (+1 more)

Nonlinear optics expands the polarization in powers of the drive and retains only the lowest-order terms. This truncation is safe at optical frequencies, where the convergence field lies far above the damage threshold. In the terahertz regime, phonon polaritons enhance the nonlinear susceptibilities by orders of magnitude, lowering the convergence field into the sub-damage range and opening a window in which the expansion fails while the crystal remains recoverable. We term this window silent breakdown. At resonance, the nearest singularities of the driven lattice mode are a complex-conjugate pair of branch points below the multistability threshold, so the response remains smooth and single-valued even though the $\chi^{\left( n \right)}$ description fails. The convergence field scales as $Q^{-3/2}$, so sharpening a resonance enhances the nonlinearity but narrows the range over which the expansion applies. It follows from the mode shift $S$ alone, $E_{\mathrm{conv}}=E_{\mathrm{ref}}(3\sqrt{3}QS)^{-1/2}$, giving 2$\sim$9 MV/cm for $\mathrm{LiNbO}_{3}$. Since resonant enhancement is central to terahertz nonlinear optics, this boundary defines the field range over which susceptibility-based descriptions are quantitatively reliable.
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All authors Yao Lu, Jiawei Zhang, Qiang Wu, Jingjun Xu

Optics · Cross-listed from quant-ph

Quantum Information-Theoretic Analysis of Optical Fiber Sensing via Stimulated Brillouin Scattering

Kiran Adhikari and Janis Nötzel

Stimulated Brillouin scattering (SBS) is a powerful mechanism for distributed sensing in optical fibers, as the Brillouin response is highly sensitive to local material parameters. Assuming the input probe is a quantum state, we develop a quantum-information-theoretic framework for SBS-based fiber sensing and intrusion detection. Starting from a linearized opto-acoustic Hamiltonian, we derive an effective input-output relation for a short fiber segment. We then model each fiber segment as a single-mode bosonic Gaussian channel and the entire fiber as a cascade of quantum channels. We then compute the quantum Fisher information for general SBS-dependent parameters and identify optimal measurement settings to achieve the quantum limit, comparing them with receiver-constrained strategies such as heterodyne detection and photon counting. In addition, we propose an adaptive approach that achieves first-order asymptotic optimality and provides a novel second-order correction to address preliminary miscalibration. As an application, we apply the framework to the fiber-tapping detection problem.
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Optics

Topologically steerable ultra-flat electro-optic combs via non-Hermitian frequency lattices

Weiwei Liu, Xiaolong Su, Bing Wang et al. (+7 more)

Electro-optic (EO) frequency combs convert microwave signals to the optical domain with deterministic repetition rates, enabling applications in optical telecommunications, microwave photonics, and LiDAR. However, generating flat EO combs has remained a long-standing challenge due to exponential decay of the spectral envelope inherent to bidirectional frequency coupling within conventional schemes. Here we reveal the inherent correspondence between flat-comb generation and the topological energy bands of non-Hermitian nonreciprocal frequency lattices, and experimentally realize ultra-flat combs with topologically controlled spectral flow. We derive the exact analytical condition for flat-comb generation, and find that it maps precisely onto the complex energy band, thereby establishing a direct connection between comb-envelope engineering and non-Hermitian topology. Experimentally, we observe an ultra-flat comb whose frequency evolution direction is governed by the topological winding and remains robust against detuning and phase fluctuations. This scheme can be further extended to long-range coupling and multi-lattice architectures, underscoring its versatility for flat dual- and multi-comb generation. Our work establishes a topologically steerable approach for achieving ultra-flat EO combs, paving the way for chip-scale spectral engineering via non-Hermitian topology.
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All authors Weiwei Liu, Xiaolong Su, Bing Wang, Yongjie Wang, Pang Fu, Wenxian Zhang, Ruiqian Cheng, Songpan Hong, Qingbin Zhang, Peixiang Lu

Thursday 8 October

Optics

Phantom Zernike Wavefront Sensor: Phase Contrast with Engineered Focal Plane Reference Speckles

Benjamin Courtney-Barrer

We introduce the Phantom Zernike Wavefront Sensor (phantom ZWFS), an engineered reference extension of phase-contrast wavefront sensing in which controlled pupil plane modulation generates off-axis coherent speckle structures that undergo localized focal plane phase shifts. These references may replace the central ZWFS reference when the on-axis PSF core is required for another optical function, such as photonic science coupling, or may augment a conventional central ZWFS response to provide additional wavefront information. Using a monochromatic scalar model, we derive the resulting carrier structure and show how the engineered references can be recovered through spatial demodulation and incorporated into calibrated reconstruction. A simple four-reference realization demonstrates that the reference geometry redistributes photon sensitivity across spatial frequency magnitude and orientation, producing mode dependent changes in linearity and cross-talk rather than a uniform improvement over a classical ZWFS. We then consider differential piston on a seven segment aperture as an application specific example. A hybrid sensor retaining the central ZWFS phase region and adding six engineered references that match the segment geometry gives similar precision near the phased state but substantially improved performance at larger piston excursions. Under the adopted photon-noise-limited benchmark, the hybrid reduces the nonlinear reconstruction error and provides better conditioned optical encoding of the piston state than an idealized vector ZWFS, allowing more precise piston estimation from the same detected photon flux at larger piston excursions.
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Optics

Theory of Disorder-Induced Multimode Coupling in a Fabry-Pérot Cavity

Wei-Kuo Li and Hsing-Ta Chen

Strong light–matter interactions have attracted significant interest in physical chemistry due to the potential for modifying molecular processes by hybridizing with cavity photons and forming polaritonic states. However, for a disordered molecular ensemble within an optical cavity capable of supporting multiple photon modes, theoretical understanding of multimode polaritons remains incomplete, particularly regarding whether and how distinct photon modes are mutually coupled through molecules. In this work, we generalize the collective mode expansion formalism to incorporate multiple photonic modes and show that spatial and orientational disorder in the light–matter coupling strength (originating from dipole orientation fluctuations and photonic field inhomogeneity) can mediate cross-mode coupling. This disorder-induced coupling allows for energy transfer to otherwise unexcited photon modes through a mechanism termed disorder-induced multimode activation (DIMA). Using an effective one-dimensional Fabry–Pérot cavity as a test system, we demonstrate that the cross-mode energy transfer through DIMA can be controlled by changing the molecular layer thickness and the optical properties of the mirrors. Furthermore, we show that DIMA is robust against weak energetic disorder. This mechanistic understanding lays the foundation for a systematic control of multimode photon coupling using disorder-induced effects.
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Optics · Cross-listed from quant-ph

Optimal linear-optical interferometers for unitary t-designs in a multi-photon Fock space

Robert A. Grinshtein, Ivan V. Dyakonov and Stanislav S. Straupe

We investigate the construction of unitary $t$-designs in a multi-photon Fock space on programmable linear-optical interferometers. Using the frame potential as the optimization objective, we identify architectures that realize approximate unitary 1- and 2-designs with considerably fewer controllable parameters than required for universal linear optics. For example, we find numerical evidence suggesting that for unitary 1-designs, the required number of programmable parameters scales empirically as $2d$ with the number of modes $d$, instead of $O(d^2)$. We benchmark the resulting ensembles using single-photon transition probabilities and average channel fidelities. Furthermore, as a practical result, we develop a photon-number estimation protocol for multi-photon Fock states that leverages these optimized 1- and 2-designs. Our findings demonstrate that shallow interferometers can accurately reproduce Haar-averaged first moments, while second-moment-sensitive observables provide a considerably stricter test for approximate unitary 2-designs.
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Optics

Coherent ultraviolet frequency combs generated by resonant dispersive waves

Michael Hemsworth, Arthur K. Mills, Christina Hofer et al. (+2 more)

Resonant dispersive wave (RDW) emission in gas-filled anti-resonant hollow-core fibers (AR-HCF) offers a wavelength-tunable route to ultraviolet (UV) and vacuum ultraviolet (VUV) frequency comb generation. RDW emission is expected to be coherent fundamentally, but various noise sources are expected to limit the coherence, including amplitude-to-phase fluctuations due to input pulse intensity noise and plasma formation. It is critical, therefore, to establish the limits of the coherence of UV RDW pulses to determine their applicability for precision spectroscopy. Here, we demonstrate the coherence of RDW emission at 260~nm generated with an amplified Yb-doped fiber system at a 2-MHz repetition rate with a self-heterodyne beat measurement between the RDW and the fourth harmonic of the source. We isolate the RDW contribution to the frequency noise by coherently subtracting the carrier-envelope offset noise contribution measured simultaneously with an f-2f interferometer. We observe a beatnote linewidth below 10~kHz over timescales longer than 100~ms, and we find a conservative upper bound of 1~rad on the integrated RMS phase deviations over a frequency range spanning 1 to 30~kHz. The dominant frequency noise contributions are consistent with environmental noise of the interferometers and AR-HCF coupling fluctuations. These results provide strong evidence that RDW emission is coherent under well-controlled experimental conditions, confirming its potential for UV and VUV frequency comb generation.
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All authors Michael Hemsworth, Arthur K. Mills, Christina Hofer, TJ Hammond, David J. Jones

Optics

Broadband soliton microcombs in the biological window enabled by a resonant coupler

Xinrui Luo, Du Qian, Yuanlei Wang et al. (+8 more)

Broadband soliton microcombs near 1 $\mu$m offer coherent, evenly spaced optical lines for measurements in the biological window. However, combining broad spectral coverage with closely spaced comb lines increases the pump-power requirement and makes thermally stable soliton access more difficult. Here, we demonstrate broadband silicon nitride (Si3N4) soliton microcombs enabled by a resonant coupler that enhances pump delivery to the nonlinear resonator. The device generates a spectrum spanning 930-1380 nm with an approximately 98-GHz comb-line spacing at on-chip pump powers of tens of milliwatts, while providing reliable thermally stable access to the single-soliton state. These results provide a route to compact, power-efficient microcomb sources that combine broad spectral coverage with closely spaced comb lines for spectroscopy and imaging in the biological window.
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All authors Xinrui Luo, Du Qian, Yuanlei Wang, Yiwen Yang, Kaixuan Zhu, Yinke Cheng, Haoyang Luo, Junqi Wang, Bei-Bei Li, Qihuang Gong, Qi-Fan Yang

Optics · Cross-listed from physics.atom-ph

Local polarization control of optical tweezer arrays

Xiangkai Sun, Richard Bing-Shiun Tsai, Andrew Winnicki et al. (+4 more)

Optical tweezer arrays trapping ultracold atoms and molecules are a versatile quantum science platform with broad impact in quantum simulation, quantum computation, and quantum metrology. The polarization dependence of anisotropic vector and tensor light shifts offers a degree of freedom for precise quantum state engineering of trapped particles. Similarly, state-selective operations depend on the polarization of the addressing light through the atom–light coupling strength. Yet, methods for controlling the polarization of individual tweezers or local addressing beams remain an experimental challenge. Here, we demonstrate independent, site-resolved linear polarization rotation across an optical tweezer array by exploiting the local birefringence tunability of a spatial light modulator. Applying this capability to an array of $^{88}\rm{Sr}$ atoms, we homogenize differential light shifts imparted by the 813-nm tweezers to the narrow ${^1{\rm S}_0}\leftrightarrow{^3{\rm P}_1}$ transition, enabling sideband cooling under a magic-angle condition. Furthermore, we show dynamic transport of trapped atoms across distinct polarization zones with high survival and preserved coherence. Finally, we characterize tweezer polarization noise and demonstrate closed-loop stabilization referenced to the atomic transition frequency, reaching mrad-level stability. Our work establishes a technique for manipulating polarization-sensitive atomic and molecular transitions in tweezer architectures, with immediate applications to optical tweezer clocks and multi-zone quantum processors.
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All authors Xiangkai Sun, Richard Bing-Shiun Tsai, Andrew Winnicki, Yuan Le, Kon H. Leung, Nelson Darkwah Oppong, Manuel Endres

Optics

Spatiotemporal computing for temporal nonlinear tasks using few-mode fiber and SOA nonlinearity

Adhila Nazarudeen, Silvia Ortín and Apostolos Argyris

Step-index few-mode fibers (FMFs) provide a compact passive platform for high-speed photonic information processing by exploiting modal dispersion to map temporal inputs into spatiotemporal representations with short-term memory. In previous studies, we have demonstrated linear classification tasks with such FMFs, such as ultrafast multibit header recognition. Here, we extend the computational capability of such architectures towards solving nonlinear tasks by introducing an optical nonlinearity at the output of the dispersive medium. The FMF output is processed through parallel direct and nonlinear branches, with the latter implemented using a nonlinear semiconductor optical amplifier (SOA). We experimentally evaluate the architecture at 28.5 Gb/s on two delayed XOR and higher-order parity tasks. We show that combining the direct and SOA-transformed representations reduces classification errors and broadens the range of operating conditions supporting low-error classification. In addition, by exploiting multiple temporal samples within each bit period as independent classifier features, we increase the dimensionality of the photonic representation and improve the computational performance.
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Optics

Harnessing Machine Learning for Ultrafast Science in Facility Settings: Opportunities and Challenges

Mousumi Upadhyay Kahaly, Ishraq Md Anjum, Davorin Peceli et al. (+2 more)

Ultrafast science is rapidly evolving from a collection of specialized laser experiments into a facility-scale, data-intensive research endeavour. High-repetition-rate femtosecond and attosecond sources, synchronized multidimensional diagnostics, and increasingly nonlinear laser-matter interactions now generate heterogeneous data streams whose volume, speed, and complexity often exceed the capacity of conventional manual workflows. This chapter examines how machine learning can become an enabling layer for next-generation ultrafast facilities, with emphasis on the Extreme Light Infrastructure (ELI) ecosystem. We discuss the scientific drivers for artificial-intelligence-assisted experimentation, propose a facility-scale digital architecture spanning acquisition, ingestion, storage, analytics, machine learning, adaptive feedback, and user interfaces, and review representative examples from ELI ALPS, ELI Beamlines, and ELI-NP. The central thesis is that machine learning should not be treated merely as an offline data-analysis tool; rather, when constrained by physical insight and embedded in robust data infrastructure, it can support real-time diagnostics, surrogate modeling, anomaly detection, experimental optimization, and progressively autonomous discovery workflows.
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All authors Mousumi Upadhyay Kahaly, Ishraq Md Anjum, Davorin Peceli, Zsolt Divek, Subhendu Kahaly

Optics

Minimizing round-trip loss for near-unity quantum efficiency in organic light-emitting diodes

Junho Kim, Seonjeong Lee, Seunghyup Yoo et al. (+1 more)

Modern organic light-emitting diodes (OLEDs) reach internal quantum efficiencies close to 100%, yet in a planar device only 20-30% of photons escape. External outcoupling structures on high-index substrates recover some of the trapped light, but external quantum efficiencies (EQEs) have stalled at 55-60% across different optical designs, and the origin of this ceiling is not fully understood. Here we identify light extraction in such OLEDs as a multi-pass process: light that the outcoupling structure fails to extract returns to the OLED, is reflected back and may escape over multiple passes, with the efficiency governed by the absorption per round trip. Absorption loss is minor in conventional planar OLEDs, but it becomes dominant and accounts for the observed efficiency ceiling when an outcoupling structure is attached. Consistent with this picture, adding a low-loss auxiliary reflector raised the EQE of an OLED on a high-index microlens substrate from 51% to 77% without any change to the OLED stack or the outcoupling structure, the highest value reported for a single-junction OLED without a macroscopic extraction lens; for an OLED on a conventional glass substrate an estimated 92% of the light delivered to the substrate was extracted. We derive four design rules that keep round-trip losses to a few percent and predict that EQEs around 90% are feasible with established materials. Under these rules, microcavity tuning and emitter orientation largely cease to matter, and high efficiency is retained across a wide range of microlens designs and scattering layers.
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All authors Junho Kim, Seonjeong Lee, Seunghyup Yoo, Malte C. Gather

Optics

Observation of robust corner states in photonic crystals without global symmetries

Zitao Ji, Jianfeng Chen, Yidong Zheng et al. (+2 more)

Robust corner states have attracted considerable attention in recent years, particularly in photonic systems. They are typically realized in second-order topological insulators, including shrunken-expanded lattices and breathing Kagome lattices, where global symmetries are essential in their formation. This reliance on symmetry, however, can make these states highly sensitive to structural disorder. Here, we propose and experimentally demonstrate a class of robust corner states in photonic crystals that require neither global symmetry nor nontrivial topology. By locally tailoring the corner geometry of an otherwise topologically trivial structure, we create strongly localized modes that remain stable under both position and radius disorder. These states are not accidental; rather they originate from a pair of Dirac points in the bulk band structure. Geometric tuning shifts the flatband edge states connecting the Dirac points into an otherwise trivial bandgap, thereby inducing strongly localized corner states. Our findings uncover a previously overlooked class of symmetry-independent, disorder-resilient corner states, extend robust localization beyond conventional topological protection, and provide a route toward light localization in all-dielectric photonic systems.
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All authors Zitao Ji, Jianfeng Chen, Yidong Zheng, Zhaogang Dong, Zhi-Yuan Li

Optics

Temporal chirality in Floquet photonic media

Rui-Chuan Zhang, Yixin Sha and Yi Yang

We reveal temporal chirality in time-modulated photonic media: reversing the orientation of a closed loop traced by the complex permittivity can leave the Floquet spectrum unchanged while strongly modifying the directional scattering response under the same excitation. This behavior is linked to two symmetry constraints on the evolution operator, namely pseudo-Hermiticity and pseudo-unitarity. We prove the general occurrence of temporal chirality based on a complex Bogoliubov parametrization of the scattering amplitudes. We illustrate these findings in a single-harmonic temporal-$\mathcal{PT}$-symmetric system: both modulation-loop orientations support the same vacuum-like Floquet photonic bands at the phase transition, but one suppresses backward photon generation whereas the other induces strong growth, as confirmed by full-wave simulations. Our results establish temporal chirality as a distinct notion in time-varying photonics and identify modulation-loop orientation as a control knob for Floquet wave dynamics.
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Optics · Cross-listed from physics.bio-ph

Femtosecond Three-Dimensional Imaging of Single-Protein with Hard X-ray Laser

Paul Lourdu Xavier, Andrew J. Morgan, Johan Bielecki et al. (+66 more)

The extremely intense pulses of X-ray free-electron lasers (XFELs) have enabled imaging of radiation-sensitive samples, such as macromolecular microcrystals, beyond radiation damage limits. These sources have the potential to deliver biomolecular single-particle imaging, similar to cryo-electron microscopy but without the need for cryo-fixation and with temporal resolution from femtoseconds to milliseconds. While this possibility was recognized before XFELs were built, the biological single-particle imaging work-flow has previously only been demonstrated on large virus particles. Based on decades of improvements in X-ray beam focusing, particle delivery, diffraction detection, and advanced analysis, here we demonstrate imaging of a single molecular complex, the giant-hemoglobin erythrocruorin (Ery) with X-ray laser pulses. Two-dimensional classes of diffraction patterns could be reconstructed to 15 Angstrom resolution, and 3D images to approximately 20 Angstrom, while the 3D merged intensity in reciprocal space extended beyond 20 Angstrom. The resolution discrepancy is likely due to heterogeneity caused by gas-phase compaction of the complexes. With increased throughput, this approach could be used to reveal in-situ structural details during mass spectrometry studies of biomolecules, while improvements in sample delivery may provide ultrafast snapshot imaging of biological single-particles in their native-state beyond the limitations of radiation damage.
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All authors Paul Lourdu Xavier, Andrew J. Morgan, Johan Bielecki, Amit K. Samanta, Simon Dold, Chufeng Li, Xinyue Gao, Spencer K. Passmore, Ruslan P. Kurta, Tim B. Berberich, Emiliano De Santis, Wenhui Zhang, Juncheng E, Faisal H. M. Koua, Romain Letrun, Luca Gelisio, Oleksii Turkot, Egor Sobolev, Lukas V. Haas, Jannik Lübke, Jingxuan He, Madeline B. F. Memovich, Kevin Janson, Stefanie Lenzen, Safi Rafie-Zinedine, Mansi Butola, Oleksandr Yefanov, Fabian Trost, Chan Kim, Tomas Popelar, Tokushi Sato, Yevheniy Ovcharenko, Rebecca Boll, Sergey Usenko, Thomas M. Baumann, Alberto De Fanis, Björn Senftleben, Matteo Porro, Andrea Castoldi, Ibrahym Dourki, Cyril Danilevski, Ekaterina Round, Christina Schmidt, Huijong Han, Roberto Alvarez, Ervin Chia, Cornelia Cazey, Carolin Seuring, Ruojie Sha, Maia Azubel, David A. Bushnell, Sébastien Boutet, Sergey Ovchinnikov, Adrian P. Mancuso, Michael Meyer, Richard Bean, Anton Barty, Filipe R. N. C. Maia, Richard A. Kirian, Thomas D. Grant, N. Duane Loh, Carl Caleman, Erik G. Marklund, Nadia Zatsepin, Andrew V. Martin, Saša Bajt, Roger D. Kornberg, Jochen Küpper, Henry N. Chapman

Optics

RF-Resolved RIN Engineering of High-Power CW Yb Fiber Laser Architectures Using Seed Topology, Seed Power, Pumping Geometry and Cavity FSR

Anitta Augustine, V. Meher, Shantanu Dambare et al. (+1 more)

We report, to the best of our knowledge, the first controlled, like-for-like experimental mapping of how four practical design parameters–seed-cavity topology, seed power, amplifier pumping geometry, and seed-cavity free spectral range (FSR)–jointly shape the radio-frequency (RF)-resolved relative intensity noise (RIN) of high-power continuous-wave (CW) Yb-doped fiber lasers. We compare a standalone bidirectional fiber Bragg grating (FBG) oscillator, an FBG-seeded master-oscillator power amplifier (MOPA), and unidirectional ring-seeded MOPAs with FSRs of approximately 11.5 and 5 MHz, all near 1064 nm at a common 72 W output, with RIN measured from 10 kHz to 1 GHz. Across all settings, the low-frequency RIN is technical-noise limited and nearly architecture-independent, whereas RIN above 160 kHz is governed by the seed-cavity topology and its transfer through the amplifier. Higher seed power suppresses the mid- and high-frequency RIN, but the seed-alone and amplified-output comparison reveals opposite transfer for the two topologies: for FBG seeding the 2 MHz-1 GHz RMS RIN increases through amplification (3.66 percent to 4.14 percent at 15 W seed), whereas for ring seeding it decreases (1.47 percent to 1.35 percent). Forward pumping gives the best power-scaling/noise trade-off, while reducing the ring FSR from approximately 11.5 to 5 MHz preserves the high-frequency RIN advantage, confirming it is not a mode-spacing effect. At matched conditions, ring seeding lowers the integrated RMS RIN by 2-3 times (160 kHz-2 MHz) and approximately 3 times (2 MHz-1 GHz, approximately 9-10 dB to near -127.4 dBc/Hz). These results yield band-resolved design rules linking each band to its dominant lever. The unidirectional ring-seeded MOPA thus offers a simple, scalable route to broadband pump-RIN suppression for nonlinear and frequency-conversion photonics.
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All authors Anitta Augustine, V. Meher, Shantanu Dambare, V. Balaswamy

Optics · Cross-listed from physics.bio-ph

Three-dimensional imaging of isolated membrane-protein complexes in vacuo with an X-ray laser

Kartik Ayyer, Tej Varma Yenupuri, Johan Bielecki et al. (+51 more)

The prospect of imaging single biomolecules, viruses and cells with intense, ultrashort X-ray pulses has driven the development of X-ray free-electron lasers (XFELs). However, the weak scattering from small particles is easily swamped by background from residual gas, which has so far limited applications to strongly scattering targets such as viruses, cell organelles and cells. Here we report a three-dimensional (3D) reconstruction of an isolated 1-MDa membrane-protein complex, photosystem I (PS I), from single-particle diffraction data. PS I trimers were aerosolised by charge-reduction electrospray ionisation and injected into the European XFEL beam, with partial helium gas exchange reducing background scattering by 80%. From 32 788 diffraction patterns of single trimers in random orientations, we reconstructed the 3D electron density to a resolution of 3.8 nm, limited by the detector geometry. The disc-shaped density, about 22 nm across and 10 nm thick, matches the size of a PS I trimer in a detergent micelle and is consistent with the compaction predicted by molecular dynamics simulations of the complex in vacuo and observed in native mass spectrometry. These results show that membrane-protein complexes can be imaged in vacuo with X-ray lasers, an important step towards ultrafast diffractive imaging of single macromolecules.
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All authors Kartik Ayyer, Tej Varma Yenupuri, Johan Bielecki, Lutz A. Eichacker, J. Domingo Meza-Aguilar, Louise J. Persson, Lena Worbs, Patrick Adams, Roberto Alvarez, Adil Ansari, Alfredo Bellisario, Rebecca Boll, Carl Caleman, Sebastian Cardoch, Andrea Castoldi, Joe P. J. Chen, Cyril Danilevski, Merle Dechering, Simon Dold, Armando D. Estillore, Alberto De Fanis, Lukas V. Haas, Patrick Konold, David Lomitze, Abhishek Mall, Dimitra Manatou, Iñaki de Diego Martinez, Parichita Mazumder, Andrew J. Morgan, Anna Munke, Yevheniy Ovcharenko, Matteo Porro, Safi Rafie-Zinedine, Amit K. Samanta, Christina Schmidt, Arezu Sehati, Jonas A. Sellberg, Björn Senfftleben, Carolin Seuring, Zhou Shen, Nicusor Timneanu, Sergey Usenko, Daniel Westphal, August Wollter, Tamme Wollweber, Paul Lourdu Xavier, Henry N. Chapman, Janos Hajdu, Richard A. Kirian, Jochen Küpper, Erik G. Marklund, Petra Fromme, Michael Meyer, Filipe R. N. C. Maia

Optics · Cross-listed from cond-mat.mtrl-sci

Predicting plasmonic pole representations of complex dielectric spectra using convolutional neural networks

Dario A. Leon

Extracting spectral properties such as energy position, broadening, and spectral weight from dielectric spectra is critical for interpreting collective electronic excitations in many-body physics. Conventional approaches typically rely on nonlinear fitting procedures that can be computationally demanding and highly sensitive to initialization and fitting choices. In this work, we develop a convolutional neural-network framework for the direct inversion of dielectric spectra into their underlying plasmonic pole structures using a multipole-Padé representation. Rather than training on a constrained database of spectra associated with a specific set of materials, the network is trained entirely on synthetic spectra generated from the analytical multipole-Padé expression with randomized parameters. This enables the network to learn the general mapping between the spectra and their features in an unbiased way, without requiring large, material-specific datasets derived from real materials. We demonstrate that the synthetically trained network generalizes to complex first-principles and experimental dielectric spectra, extracting the underlying pole parameters with high accuracy in a single forward pass. This approach provides an efficient and robust alternative to conventional nonlinear fitting, enabling high-throughput, automated analysis of dielectric spectra across diverse materials and applications.
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Optics · Cross-listed from quant-ph

From Solid-State Spin Defects to Fluorescent Proteins: A Perspective on Quantum Optical Sensing Platforms for Biomedical Applications

Jyotiprakash Parhi, Ayan Majumder and Kasturi Saha

Quantum optical sensing has evolved from isolated solid state spin defect platforms to increasingly bio-integrated systems designed for biomedical applications. This review presents a perspective on this transition, spanning nitrogen vacancy centers in diamond to emerging fluorescent protein based spin qubits. Advances in diamond fabrication have enabled photonic cavities, nanomechanical resonators, and microstructured devices that enhance spin photon interactions and room temperature sensitivity. Exploration of spin defects in alternative materials further expands the sensing landscape. Parallel progress toward biological compatibility includes chip based architectures for in vitro and in vivo studies, microfluidic integration, and minimally invasive nanodiamond probes capable of intracellular nanothermometry and nanorheometry. Improvements in surface functionalization have enhanced coherence and sensing reliability in complex environments. Collectively, these advances highlight the progress and remaining challenges toward application oriented quantum sensing for the life sciences.
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Optics

Undetected Photon Spectroscopy in the Long-wave and Far Infrared

Jinghan Dong, Jingrui Zhang, Haichen Zhou et al. (+3 more)

Spectroscopy with undetected photons allows the spectral response of a sample at long wavelengths to be measured through interference of a shorter-wavelength signal field, without directly detecting the light that probes the sample. Previous demonstrations of undetected-photon spectroscopy in the molecular fingerprint region have reached approximately 10.5 microns, leaving longer wavelengths largely unexplored. Here, we extend this approach to 20 microns wavelength, demonstrating infrared undetected-photon spectroscopy using a BaGa2GeSe6 (BGGSe)-based nonlinear interferometer while only the near-infrared (NIR) signal is detected using a silicon detector. Rotation of the BGGSe crystal provides broad wavelength tuning, and Fourier-transform analysis of the signal interferograms yields the corresponding idler spectra and sample transmittance. Broadband absorption spectroscopy is demonstrated using Kapton across a 7.8-20 microns wavelength range, with the measured absorption agreeing with independent infrared measurements. The same system is further employed to detect sulfur hexafluoride (SF6) gas via spectroscopy near 10.55 microns. These results substantially extend the accessible spectral range of spectroscopy with undetected photons while retaining silicon-based NIR detection.
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All authors Jinghan Dong, Jingrui Zhang, Haichen Zhou, Weijie Nie, John G. Rarity, Alex S. Clark